Method and device for measuring the concentration of a detector gas in a measuring gas containing an interfering gas

ABSTRACT

A concentration of a detector gas in a measuring gas containing an interfering gas is measured. A zero gas that is free of the detector gas and/or a test gas having a known concentration of the detector gas are supplied to an analyzer such as a non-dispersive infrared (NDIR) gas analyzer. Interfering gas is added to the measuring gas, the zero gas, and in some instances, to the test gas to an extent which allows the interfering gas to have a same concentration in each case.

In non-dispersive infrared (NDIR) gas analysis, the problem of crosssensitivity arises when the measuring gas not only contains the detectorgas, whose concentration in the measuring gas is supposed to bemeasured, but also contains an interfering gas, whose spectralabsorbance bands substantially overlap those of the detector gas. Thishappens, for instance, when nitrogen oxides are measured in exhaustgases, which contain water vapor as an interfering gas. To remove watervapor and other interfering gases as well, the book Messen, Steuern undRegeln in der chemischen Technik [Measuring, Controlling and Adjustingin Chemical Technology], 3rd edition, volume II, p. 611, Springerpublishing house, 1980, discloses cooling the measuring gas andsubsequently directing it over a drying agent that has to be renewedfrom time to time.

To measure nitrogen oxides in the exhaust gas of motor vehicle engines,the International Patent Application W0 89/03029 also discloses coolingthe measuring gas in order to remove the water vapor contained in thegas and, in this manner, to facilitate the nitrogen oxide measurement.The disadvantage of the known measures for removing water vapor is thatan expensive cooler is needed, which is subjected to wear, and thatoperating and maintenance costs accrue.

The underlying object of this invention is to indicate a method and adevice for measuring the concentration of a detector gas in a measuringgas containing an interfering gas, which is simpler than known methodsand will ensure better operational reliability.

The idea behind the invention is to add so much interfering gas to themeasuring gas, the zero gas, and the test gas that the same interferinggas concentration will be reached in each case. When the originalconcentration of interfering gas in the measuring gas is more or lessthe same or is higher, it can be necessary to lower the concentration bymeans of cooling. For this purpose, it generally suffices to simply coolthe measuring gas to about the ambient temperature. It is beneficial tomore or less retain the original concentration of the interfering gas orto only change it slightly, because any such change also affects theconcentration of the detector gas, and this generally has to be allowedfor in a corrective calculation.

The measured value determined with the zero gas corresponds to a zeroconcentration of the detector gas and, therefore, constitutes the zeropoint. This value is subtracted from the measured values of themeasuring gas and, in fact, either arithmetically or, in the case ofdouble-beam NDIR analyzers, physically. When the test gas, whichcontains the detector gas in a known concentration, is measured, theincrease in the measuring effect is determined.

In the case that the concentration of nitrogen oxides inwater-vapor-containing exhaust gas is measured using a single-beam NDIRanalyzer, the measuring, zero, and test gases are directed one afteranother through a single humidifier. This has the advantage that thegases are moistened in the same manner. However, a measuring pause mustbe observed from time to time to redefine the zero point and theincrease. This generally does not interfere, since when measuringexhaust gases from motor vehicle engines, for example, the measuringpauses that are required anyway can be utilized. In the case of adouble-beam analyzer, the zero gas can be used as a reference gas, sothat the zero point does not need to be determined separately. In thiscase, however, two humidifiers are needed, preferably with a combinedconstruction, so that largely conforming conditions, for example thesame temperatures, will prevail for the moistening operation.

In one advantageous specific embodiment, the humidifier contains ahumidifying chamber, through which the gases are directed and which hasa moistening wall made of a material, which absorbs the water andreleases it to gases that are not saturated with water vapor. In onespecific embodiment, which is especially simple, but is less suited fora higher measuring accuracy, the moistening wall removes water vaporfrom the measuring gas and releases it again to the zero gas and thetest gas. In this manner, one compensates for the originally variablewater-vapor content of the gases, for example of the exhaust gas,ambient air and test gas from a bottle. The moistening wall ispreferably brought into contact on its rear side with water in liquid orvaporous form, which it absorbs and releases to the gas flowing throughthe moistening chamber.

On the basis of the drawing, the invention, as well as furtheradvantages, developments and additions are described in greater detailand clarified in the following.

FIG. 1 schematically depicts an arrangement for measuring theconcentration of nitrogen oxides in exhaust gases.

FIG. 2 shows a gas humidifier in cross-section, which can be usedalternatively in the arrangement according to FIG. 1.

Exhaust gas from a motor vehicle engine is supplied to the arrangementshown in FIG. 1 via an inlet (entry site) E. The exhaust gas cools offin the line and arrives in a condensation separator (settling) tank K,at whose base the condensate water collects. The exhaust gas liberatedfrom the excess water vapor flows through a filter F and through a valveV1 to a pump P1, which delivers the exhaust gas through a valve V2 intoa moistening chamber BK1 of a humidifier B. From there, the gas arrivesin an NDIR analyzer, which is used to measure the nitrogen-oxideconcentration of the exhaust gas.

Situated above the moistening chamber BK1 in the humidifier B is awater-supply chamber WK1, into which a pump P2 pumps condensate waterout of the condensation separator (settling) tank K, which collectsthere over a moistening wall BW1. This consists of a material, whichabsorbs water and releases it again, provided that it is circumflowed bygas that is not saturated with water vapor. In the exemplary embodiment,polyamide is used as such a material, which is reinforced with glassfiber to increase strength. The wall should be very thin, preferably afoil with a thickness of less than 1mm. Of course, the remaining partsof the humidifier B can also consist of this material. Also, thehumidifier can be constructed so as to allow water to flow completelyaround a moistening chamber.

The measuring gas flowing through the moistening chamber BK1 absorbswater vapor from the moistening wall BW1 up to a specific concentrationessentially given by the ambient temperature. Of course, the residencetime in the moistening chamber BK1 must be long enough. Thus,interfering gas is added selectively to the measuring gas.

To compensate for the measuring effect caused by the interfering gas inthe analyzer AG, the valve V1 is switched over during a measuring pauseand ambient air is delivered by the pump P1 through the moisteningchamber BK1 to the analyzer AG. This air absorbs moisture, as does themeasuring gas.

Since it is essentially free of nitrogen oxide, it is solely theabsorbed water vapor that determines the measuring effect. Therefore, ifone subtracts the measured value determined in this case from themeasured values acquired during the measurement of the exhaust gas, as adifference, one obtains a value, which corresponds to the concentrationof the nitrogen oxide.

A vessel PG contains test gas, which in the present case is nitrogenoxide in a known concentration. With its help, a second point of themeasuring characteristic of the measuring arrangement can be determined,in that the valve V2 is switched over and the test gas is suppliedthrough the moistening chamber BK1 to the analyzer AG. Also, the testgas absorbs moisture in this case and, in fact, in the sameconcentration as the measuring gas and the zero gas. While the measuredvalue of the zero gas yields the zero point of the measuringcharacteristic, the increase in the measuring effect is determined fromthe measured-value test gases.

To allow the gases flowing through the moistening chamber BK1 to absorbsufficient moisture, it might be necessary to increase the surface areaof the moistening wall BW1, for instance, by roughening it. FIG. 2 showsanother specific embodiment, where the moistening wall has a largesurface area, in which a cross-section through a humidifier is shown.Its moistening wall BW2 has a meander-shaped design. It divides thehumidifier into a moistening chamber BK2 and a water-supply chamber WK2.There is water W on the side of the water-supply chamber WK2 in theslits formed by the meander shape; the gas to be moistened flows throughthe meander-shaped slits inside the moistening chamber BK2. In place ofthe meander shape, any other form that enlarges the surface area canalso be selected.

What is claimed is:
 1. A method for measuring a concentration of adetector gas in a measuring gas containing an interfering gas,comprising steps of:supplying at least one of a zero gas that is free ofthe detector gas and a test gas having a known concentration of thedetector gas to an analyzer; and adding interfering gas to the measuringgas and to the zero gas to an extent such that a concentration of theinterfering gas in the measuring gas is equal to a concentration of theinterferinggas in the zero gas.
 2. The method according to claim 1,further comprising steps of:partially removing interfering gas from themeasuring gas; and adding interfering gas to the partially removedmeasuring gas.
 3. The method according to claim 1, wherein theinterfering gas is water vapor, and further comprising a step ofhumidifying or moistening the measuring gas, the zero gas, and the testgas.
 4. The method according to claim 1, wherein said adding step addsinterfering gas to the test gas.
 5. The method according to claim 4,further comprising steps of:partially removing interfering gas from themeasuring gas; and adding interfering gas to the partially removedmeasuring gas.
 6. The method according to claim 4, wherein theinterfering gas is water vapor, and further comprising a step ofhumidifying or moistening the measuring gas, the zero gas, and the testgas.
 7. A device for measuring a concentration of a detector gas in ameasuring gas containing an interfering gas, comprising:means forsupplying at least one of a zero gas that is free of the detector gasand a test gas having a known concentration of the detector gas to ananalyzer; and means for adding interfering gas to the measuring gas andto the zero gas to an extent such that a concentration of interferinggas in the measuring gas is equal to a concentration of interfering gasin the zero gas.
 8. The device according to claim 7, further comprisinga humidifier through which the measuring gas and the zero gas aredirected.
 9. The device according to claim 8, wherein the humidifiercontains a moistening chamber through which the measuring gas and zerogas are directed and which has a moistening wall made of a material,which absorbs water and releases it to gases that are not saturated withwater vapor.
 10. The device according to claim 9, wherein on the sidefacing away from the moistening chamber, the moistening wall is incontact with water.
 11. The device according to claim 9, wherein themoistening wall consists of glass-fiber-reinforced polyamide.
 12. Thedevice according to claim 9, wherein the humidifier has a water-supplychamber which is separated from the moistening chamber by the moisteningwall.
 13. The device according to claim 12, wherein the water-supplychamber is situated above the moistening chamber, and the moisteningwall lies in a horizontal plane.
 14. The device according to claim 9,wherein the moistening wall has an enlarged surface area which isgreater than a surface area of a plane encompassing the enlarged surfacearea of the moistening wall.
 15. The device according to claim 14,wherein the moistening wall has one of a meander shape and a roughsurface.
 16. The device according to claim 9, wherein water supplied tothe moistening wall is condensate water from the measuring gas.
 17. Thedevice according to claim 7, wherein said means for adding addsinterfering gas to the test gas.
 18. The device according to claim 17,further comprising a humidifier through which the measuring gas, thezero gas and the test gas are directed.
 19. The device according toclaim 7, further comprising:means for partially removing interfering gasfrom the measuring gas; and means for adding interfering gas to thepartially removed measuring gas.
 20. The device according to claim 18,wherein the humidifier contains a moistening chamber through which themeasuring gas, test gas and zero gas are directed, and which has amoistening wall made of a material which absorbs the water and releasesit to gases that are not saturated with water vapor.
 21. The deviceaccording to claim 20, wherein on the side facing away from themoistening chamber, the moistening wall is in contact with water. 22.The device according to claim 20, wherein the moistening wall consistsof glass-fiber-reinforced polyamide.
 23. The device according to claim20, wherein the humidifier has a water-supply chamber which is separatedfrom the moistening chamber by the moistening wall.
 24. The deviceaccording to claim 23, wherein the water-supply chamber is situatedabove the moistening chamber, and the moistening wall lies in ahorizontal plane.
 25. The device according to claim 20, wherein themoistening wall has an enlarged surface area which is greater than asurface area of a plane encompassing the enlarged surface of themoistening wall.
 26. The device according to claim 25, wherein themoistening wall has one of a meander shape and a rough surface.
 27. Thedevice according to claim 20, wherein the water supplied to themoistening wall is condensate water from the measuring gas.